Many underground injuries are caused by the detachment and falling of rock fragments from rock masses in mines, which are unstable. For this reason it is considered important to have a means whereby the integrity, i.e. the stability or otherwise, of a rock mass can be assessed before mine workers enter the mine working and are exposed to potential injury from falling rock fragments.
A technique which has been in use for many years to assess the integrity of a rock mass in a mine working, typically the hanging wall, involves tapping the rock mass with a sounding bar, listening to the sound generated and making an assessment of the integrity of the mass according to the sound which is heard. The sound which is heard is caused primarily by the acoustic wave generated through vibration of the rock mass and other sources, for example the sounding bar, in the surrounding environment. The sound has a unique frequency distribution which must be interpreted in order for a determination to be made of the integrity of the rock mass.
Experienced miners familiar with this technique know that an intact rock mass, i.e. a rock mass which is sufficiently stable to be regarded as safe, will respond to the applied tapping with a relatively high frequency sound, while a detached rock mass, i.e. a rock mass which is insufficiently stable to be regarded as safe, will respond to the applied tapping with a relatively low frequency sound.
Accordingly, if an experienced miner hears a low frequency response, he will categorise the rock mass in question as detached or unsafe, and will usually stipulate that the rock mass must be barred down to detach loose fragments before the area can be declared safe.
The decision making process, i.e. the assessment of the rock mass as safe or not, is however subjective and errors can be made. Such errors may for instance be attributable to the ambient noise level, personal fatigue, inexperience, and the hearing ability of the person making the assessment.
In an effort to provide a more objective assessment technique, U.S. Pat. No. 4,598,588 proposes a detached rock evaluation device in which the rock mass is struck with a sounding bar. The acoustic signal which is generated is captured by means of an accelerometer in contact with the rock mass, and the captured signal is passed through a high frequency band pass filter arranged to pass acoustic signals with a frequency in the specific range of 3000 Hz to 3500 Hz and a low frequency band pass filter with a frequency in the specific range of 500 Hz to 1000 Hz. The signals passed by the band pass filters are compared by a differential amplifier. On the assumption that a detached rock mass will generate an acoustic signal predominantly in the said specific low frequency range and that an intact rock mass will generate an acoustic signal in the said specific high frequency range, the differential amplifier will detect a small amplitude difference between the outputs of the band pass filters for an intact rock mass and a greater amplitude difference for a detached rock mass. Depending on the amplitude difference detected, the differential amplifier outputs a signal indicative of the integrity of the rock mass.
A problem with the proposal in U.S. Pat. No. 4,598,5887 is that it is only suitable for use with rock masses that generate acoustic signals within predetermined frequency ranges whereas in reality the geotechnical properties of rock masses differ vastly from one area to another, to the extent that actual frequency responses may fall outside these ranges. The device described in the US patent is accordingly unsuitable for application in a variety of areas having different geotechnical properties.
Another proposal intended to provide a more objective assessment of the integrity of a rock mass in a coal mine is the so-called Acoustic Energy Meter (AEM) designed by Rock Mechanics Technology Limited. The AEM is placed in contact with the rock mass and a hammer is used to apply an impact to the rock mass. The AEM detects the acoustic response and assesses the integrity of the rock mass on the basis of the rate of decay of the acoustic signal. While this technique may perform well for a detached rock mass forming a simple beam structure, such rock masses are rarely encountered in actual practice and particularly in hard rock environments such as those present in gold and platinum mines.